Continuous propane dewaxing



June 25, 1935. H, v ATwELL 2,006,011

CONTINUOUS PROPANE DEWAXING Filed April 24, 1933 2 SheetsSheet lINVENTOR Jfarold Vfltwell ATTORNEY June 25, 1935. v v ATWELL 2,006,011

CONTINUOUS PROPANE DEWAXING Filed April 24, 1933 2 Sheets-Sheet 2 W/kwINVENTOR JiQTOld VfitWG/U/ 201% if? ATTORNEY Patented June 25, 1935UNITED STATES CONTINUOUS PROPANE DEWAXING Harold v. Atwell, WhitePlains, N. Y.,' assignor to Standard Oil Company, Chicago, 111., acorporation of Indiana Application April 24, 1933, Serial No. 667,564

7 Claims.

This invention relates to the separation of wax from oil by the use of aliquefied normally gaseous diluent, such as propane, and it pertainsmore particularly to a method and means whereby shock chilling may beavoided in a continuous refrigeration system.

The object of my invention is to avoid the use of batch chillers and atthe same time to avoid shock chilling which has been inevitably theresult of all continuous chillers heretofore proposed. In practicing thedewaxing process a wax-bearing oil must be dissolved in the liquefledgas at relatively high temperature under superatmospherlc pressure andthe pressure is then reduced so that the chilling will be effected byutilizing the heat of vaporization of the liquefied diluent. 'If theoil-wax solution in liquefied diluent is passed from a highpressure-zone to a low pressure zone through a valve or restrictedorifice the instantaneous drop in temperature causes the precipitationof wax which is so finely divided or is of such physical character thatit cannot be readily filtered, settled or centrifuged; it causes the waxto separate in such fine particles that the waxy oil becomes a mushy orunctuous mass of substantially uniform consistency instead of wellcrystallized wax particles suspended in diluted oil. The object of myinvention is to provide a continuous system of chilling the diluted oilby auto refrigeration (vaporization of diluent) without shock chilling.In practicing my invention I employ an insulated pipe coil for theadiabatic expansion of the refrigerating medium. By proper choice oflength and cross section of such a pipe coil, the rate of cooling amixture pumped through it can be fixed at any desired value for anygiven rate of flow. Thus, the mixture may be passed continuously throughan elongated pipe coil of gradually increasing diameter so that thetemperature of the liquid flowing through the coil will be graduallylowered at the rate of about one to eight degrees per minute, preferablyabout three or four degrees per minute, without at any time subject ingthe liquid to shock chilling. Instead of using an elongated pipe ofgradually increasing cross section I may employ a cooler in which thepipe is successively manifolded to increase the effective crosssectional area.

I may obtain the pressure drop necessary to effect the desired coolingby merely using an elongated pipe of relatively small diameter andextremely long length so that the decrease in pressure is due to thepressure drop which is, in; turn, due to a friction of flow through thepipe. The temperature of liquid propane or any other normally gaseousdiluent will necessarily fall with a fall in pressure; in other words,if the pressure at the discharge end of the elongated pipe is reduced toatmospheric, then the temperature of the propane mixture must be about40 F.- Suflicient propane must be employed in this case toabsorb byvaporization not only the frictional heat developed, but to lower thetemperature of the mixture the desired 10 amount. In such a system Iprefer to trap out propane vapor at intervals to prevent unduly highvelocities in the cooling pipe, but it should be emphasized that thecooling is effected in the pipes instead of in the traps, there being norestricted orifices or valves for causing the pressure in the traps tobe materially different from that in the pipes immediately before orafter said traps.

The invention will be more clearly understood 0 from the followingdetailed description of a preferred embodiment taken in connection withthe accompanying drawings which form a part of the specification, and inwhich- Figure 1 is a diagrammatic plan of my improved system; I

Figure 2 is a detail illustrating a manifolded pipe cooler;

Figure 3 is a modification of the pipe cooler of gradually increasedcross sectional areas along the length thereof.

The invention will be described as applied to the dewaxing of S. A. E.50 midcontinent distillate untreated lubricating oil stock. It should beunderstood, however, that the invention is equally applicable to thedewaxing of any waxbearing oil whether it is an overhead stock or aresiduum, whether it is acid treated or untreated, and whether or not ithas been treated with selective solvents, such as dichlorethyl ether,nitrobenzene, chlor-aniline, sulfur dioxide and benzol, etc. As adiluent, I will describe the use of propane which contains slightamounts of isobutane, butane, ethane and unsaturated hydrocarbons ofabout the same boiling range. The invention is equally applicable to theuse of any normally gaseous diluent, and it is specially applicable tothe use of normally gaseous ethers, halogenated compounds such as methylchloride, and to slightly miscible or immiscible refrigerants, such asammonia and carbon dioxide.

About six to ten volumes of propane from storage tank IU are passedthrough line H to mixer l2, wherein it is intimately mixed with onevolume of wax-bearing oil from pipe IS. The mixture should be at such atemperature that both the oil and the wax will be completely dissolvedin propane and, if necessary, an auxiliary closed steam coil or otherheating means may be inserted in the mixer or in the propane and/or oillines. Assuming that the mixture is in complete solution at F., the gagepressure may be about 155 pounds, and the solution at this temperatureand pressure is introduced by pipe |4 into continuous pipe cooler I5.This pipe may be extremely long and of rather small cross section sothat as the mixture flows through the pipe the pressure of the liquidwill be gradually decreased. The use of such a system, however, willnecessarily require additional cooling to compensate for the friction offiow through the pipe, and the liquids at the discharge end of the pipewill have a high velocity which may be detrimental to the physicalstructure of the wax crystals. I may balance the frictional heat bycirculating a cooling fluid around the pipes at a temperature sufiicientto counteract the frictional heat but insufficient to effect anyappreciable transfer of heat from the liquid in the pipes. If heat isabstracted from the waxy oil solution through the pipes there will be atendency for the wax to adhere to the pipes and there will likewise be atendency for the formation of an undesirable crystal structure, and theoutstanding feature of my invention is the continuous cooling of thewaxy mixture by virtue of the pressure drop within the pipe and not byvirtue of the temperature differential between the inside and outside ofthe pipe.

As above stated, the use of an elongated pipe of small cross sectionalarea to obtain the pressure drop is open to some objections and I therefore prefer to employ continuous pipe chillers of the type illustratedin Figures 2 and 3. In these modifications the effective cross sectionalarea of the pipe is gradually increased so that adiabatic expansion ofthe propane is permitted without appreciably changing the velocity offlow through the pipe. By regulating the flow through the pipe theamount of propane vaporized is regulated which, in turn, regulates therate of cool ing and in this way the temperature of the mixture may begradually reduced at about three or four degrees per minute as thesolution fiows through the pipe. In Figure 2 pipe I5 is connected bymanifold Hi to pipes l1 and I8 which is, in turn, connected by manifoldl9 to pipes 20, 2|, 22, etc. If the pressure of mixture entering pipe I5is 155 pounds gage it may be 135 gage at manifold I6, 115 pounds gage atmanifold I9, pounds gage at manifold 23, 80 pounds gage at manifold 24and 70 pounds gage at manifold 25. Alternatively, as disclosed in Figure3, a single elongated pipe |5' of gradually increased cross sectionalarea, may be employed so that as the liquid flows along the pipe moreand more propane will be vaporized, the temperature will be graduallylowered, and the vaporized propane will fiow along through the enlargedpart of the pipe. In any case, the pressure of the'propane-oil-waxmixture is reduced in pipe chiller l5 to about 10 pounds gage, at whichpressure the mixture will be at a temperature of about 40 F. By thistime a. relatively large volume of propane gas will have accumulated andI therefore introduce the mixture into a gas separation trap 26, inwhich the gases are withdrawn through pipe 21 by means of compressor 28,the compressor being driven by motor 29 which is regulated by thepressure in trap 26 so that a pressure of about 16 pounds gage ismaintained in this gas separation trap.

The liquid propane-oil-wax mixture is then passed through another pipecooler 30, which is identical in structure and in operation to coolerI5. In this case the pressure drop from one end of the pipe cooler tothe other may be about 42 pounds gage so that the temperature ofpropane-oil-wax mixture leaving the pipe cooler will be about 0 F. Hereagain it is desirable to remove accumulated propane vaporized and Itherefore pass the mixture into a second gas liquid separator trap 3|,the propane being withdrawn through pipe 32 by means of compressor 33driven by motor 34, which is regulated by the pressure in trap 3|. Thecold slurry in the bottom of trap 3| is then passed through a thirdcontinuous pipe chiller 35 which is designed along the same lines andoperates on the same principles as the chillers l5 and 30. In pipechiller 35 the pressure is dropped from 28 pounds gage to about fourpounds gage so that at the discharge of this pipe the propane oil-waxmixture will be at a temperature of about 40 F. Here again the mixtureis passed into a trap 36, which serves the double function of a gasseparatorand a storage tank for feeding the filter press. Propane isremoved from the top of tank 36 through pipe 31 by means of compressor38 driven by motor 39, the motor being regulated by the pressure in trap36. Liquid slurry from the base of tank 36is forced by pump 40 into acontinuous rotary filter press 4|, the filtrate being continuouslyaccumulated in tank 42 and the wax being continuously forced by screwconveyor or pump 43 into drum 44. It should be understood that insteadofusing a continuous filter I may use a series of batch filters anddirect the slurry consecutively from one filter to another while theremaining filters are being washed, discharged or repaired.

The filtrate from tank 42 is forced by pump 45 to stripper tower 46which is heated by steam coil 41, the propane being driven out throughpipe 48 and the finished oil being withdrawn through pipe 49 toatmospheric or vacuum stripper 50 which may likewise be provided with asuitable heating element 5|. The finished oil is withdrawn by pipe 52and pump 53 to a suitable storage.

In a similar manner propane is removed from Wax in drum 44 by means ofsteam heater 54, the propane being removed through pipe 55 and the hotwax being withdrawn through pipe 56 and flashed through reducing valve51 into low pressure stripper 58. The finished wax from this stripper iswithdrawn through pipe 59 and pump 66 to suitable storage. Propane fromstrippers 50 and 58 are removed by pipes 6| and 62 respectively, pickedup by compressor 63 and introduced, together with propane, from pipes 48and 55 into pipes 64 and 65, thence into condenser 66 in which propaneis liquefied and from which it is returned to storage tank l0.Compressors 28, 33 and 38 likewise discharge propane into pipe 65.

It will be noted that each of these compressors operates continuouslywith a uniform load and a constant pressure differential. This is animportant improvement of my continuous process over the batch coolingprocess heretofore employed because in the batch processes thecompressors must operate over a wide pressure range and must be adjustedwith the decreasing temperatures to make up for the increase in thevolume of vapors which must be removed. Such variable compressors mustnecessarily be inefiicient over the greater part of the cooling range,

they require unduly high initial costs for equipment and unduly highoperating expenses due to greater power consumed. By operatingcontinuously with uniform load and uniform pressure drop compressors 28,33 and 35 may be relatively small and they will require a minimum amountof power and attention.

In practicing my invention as above described, I found that whileallowing about 15 minutes for passage through each cooling pipe, (atotal time of transfer of about 45 minutes) I obtained a final waxslurry in which the proportion of oil to propane was about one to three.I obtained a filter rate of about .4 gallons of undiluted heavy oil persquare foot per hour and a cold test on the finished oil of about zeroto five degrees Fahrenheit. These results, however, were obtained by theuse of pipe coolers of uniform effective cross sectional area, and it isapparent that the modifications of the chiller shown in Figures 2 and 3would make it possible to employ more rapid chilling rates and at thesame time obtain higher filter rates and lower cold tests.

In the accompanying drawings I have shown three separate traps fortrapping off propane vapors. It should be understood that I may dispense entirely with these intermediate propane traps or I may greatlyincrease the number thereof. The important thing is to obtain thedesired wax crystal formation by the gradual reduction of temperature ina pipe cooler due to adiabatic expansion of a portion of the diluent dueto the pressure drop in the pipe. It should be particularly noted thatat no time is the propane oil-wax mixture passed through a valve orrestricted orifice to a zone of materially lower pressure, nor is itsubjected to unduly turbulent agitation from throttling, etc. I preferto insulate the pipe coolers and prevent any heat exchange between themand external substances. I may, however, as hereinabove pointed out,enclose the pipes in housings or heat exchangers so that I may cool theoutside of the pipes sufficiently to compensate for the heat of frictiongenerated by the flow of fluids in the pipe. For this purpose I preferto employ the cold propane vapors from the succeeding vapor trap,passing them around the coil countercurrently to the flow of solutionwithin.

As examples of other diluents equivalent to propane I may employ butane,butylene propylene, dimethyl ether, dichlor difluor methane, methylchloride, methyl formate and the like. Although I do not use heatexchangers for chilling the waxy 'oil mixture and solidifying the wax,it should be clearly understood that heat exchangers are employed forutilizing the available refrigeration of the system; for instance, aheat exchanger may be employed in the line between pump 45 and strippingtower l6, and the refrigeration value of the cold filtrate may be usedto cool wash propane (fresh propane used for washing filter cake). Ifdesired, this heat exchanger may be employed to condense and/or coolpropane vapors from any of the compressors hereinabove mentioned,thereby effecting a considerable saving in the required amount ofcondenser water.

One of the outstanding advantages of a continuous process of this typeis the possibility of employing countercurrent heat exchangers invarious parts of the system, since the temperatures at any one point aresubstantially constant at all times.

While I have described a preferred example of my invention it should beunderstood that I do not limit myself to the details hereinabove setforth except as defined by the following claims:

I claim:

1. In the method of dewaxing oils by means of liquefied normally gaseousdiluents, the step of continuously chilling a diluted oil-wax mixturewithout passing said mixture through a restricted opening between a zoneof high pressure and a zone of low pressure, which step comprisescontinuously passing said mixture through an elongated conduit wherebythe rate of vaporization of propane is controlled by the gradualpressure drop incident to the flow of said mixture through said conduit.

2. The method of obtaining readily filterable wax crystals from a waxylubricating oil stock which comprises dissolving said stock in an amountof liquefied normally gaseous diluent sufficient to supply both thediluent and refrigerant, passing said mixture through an elongatedinsulated conduit while gradually decreasing its pressure wherebyrefrigeration is gradually effected by direct vaporization of diluentfrom the mixture, removing the vaporized diluent from the resultingcooled diluted wax slurry, filtering the wax from the diluted oil, andseparating the diluent from the wax-free diluted oil.

3. The method of continuously cooling a mixture of waxy oil with anormally gaseous diluent whereby separable wax crystals are formed,which method comprises passing said mixture through an elongated closedpassageway to permit a gradual reduction of pressure with the consequentvaporization of diluent, trapping out liberated diluent vapors,continuing the flow of said mixture through an elongated closed passagefor the further gradual vaporization of diluent and again trapping outliberated diluent vapors, there being no substantial difference in thepressure between trapout zones and the elongated closed passagewaysleading to and from said zones so that the major cooling is effected inthe coils rather than in the trapout zones.

4. The process of continuously cooling a propane-oil-wax mixture toeffect the solidification of wax in readily separable form, whichcomprises forcing said mixture through an elongated closed passageway ofnarrow cross section to effect a gradual and substantial pressure drop,allowing the gradual pressure drop to cause vaporization of propane forcooling the mixture to solidify the wax and maintaining the walls of thepassageway at a sufficiently high temperature to prevent wax crystalsfrom adhering thereto.

5. The method of continuously dewaxing oil by means of a liquefiednormally gaseous diluent,

which comprises admixing a relatively large.

amount of said diluent with said waxy oil at a temperature sufficientlyhigh to effect complete solution of both oil and wax, passing saidsolution through an elongated closed passageway whereby the pressure isgradually reduced and thereby permitting a gradual vaporization ofdiluent and the gradual cooling of the solution, separating thevaporized diluent from the cooled, diluted oilwax mixture, mechanicallyseparating the wax from the diluted oil, and separating the wax-free oilfrom the diluent.

6. The method of claim 5 wherein diluent vapors are removed at aplurality of spaced points between the introduction of the mixture intothe elongated closed conduit and the mechanical separation of wax fromdiluted oil.

'7. The method of continuously cooling an oilwax mixture diluted with anormally gaseous d11- uent for obtaining readily separable wax crystals,which comprises passing said mixture through an elongated, closedpassageway, and removing vaporized diluent from the liquid at spacedpoints in said passageway by means of compressors operating atsubstantially constant pressure differentials, the pressure drop beinggradually efaooaoi 1 fected throughout the entire elongated, closedpassageway and the compressors serving to force vapors from. the trapagainst a higher pressure in a gas line whereby the major part of thecooling is efiected in the elongated conduit instead of in the traps.

HAROLD V. ATWELL.

